The present invention relates to an electric field absorption light modulation driving apparatus and method. Very fast information transmission and information transmission over a long distance are important objects achieved by a current optical communication system. For this purpose, an external modulation method instead of a direct modulation method, which may cause, for example, charping phenomena occurring in a distributed feedback laser, is required. An electric field absorption light modulation device (this is referred to as "EAM" hereinafter) is one such method fulfilling the requirement. An EAM constructed with an integrated light emission source is expected to bring about miniaturization and high performance in an apparatus relevant to the system.
The composition of an EAM driving apparatus of a related art is described below with reference to FIGS. 1A through 1C. The EAM driving apparatus comprises a laser diode device (this is referred to as "LD device" hereinafter) 1, a modulation device 2, EAM 3, a reference voltage terminal 4, a bias terminal 5, and a driving terminal 6. The LD device 1 provides a LD device output 7 to the modulation device 2, and the modulation device 2 provides a modulation device output 8.
An input-output characteristic of the modulation device 2 is described below with reference to FIGS. 2A through 2C. As is apparent from a curve (C) of FIG. 2A, the light transmittance is approximately 100% when the absolute value of the driving voltage (negative voltage) applied to the terminal 6 is zero (0) (point (C1)). Most of the light of the LD device output 7 applied to the modulation device 2, passes through modulation device 2, and appears as modulation device output 8 without experiencing significant attenuation. The LD device output 7 begins to be absorbed by the modulation device 2 at the point (C2) wherein the absolute value of the negative driving voltage begins to increase. Finally, all of the LD device output 7 is absorbed by the modulation device 2 so that the modulation device output 8 is zero (0), that is, the light is off, at the point (C3) when the absolute value of the negative driving voltage becomes equal to a predetermined value.
Therefore, an electrical driving waveform (in the negative driving voltage applied to the modulation device 2) such as shown in FIG. 2B results in an optical output waveform (in the modulation device output 8) such as shown in FIG. 2C in a case where the LD device output 7 supplied to the modulation device 2 is kept at a constant value.
There may be a case where the driving voltage applied to the modulation device 2 fluctuates in a range a delta-V.sub.H at the upper voltage (that is, small absolute value of a negative voltage) part of the driving waveform shown FIG. 3B. Such fluctuation may occur because of, for example, the high harmonics present in such a pulse wave. In the above mentioned case, the optical output (modulation device output 8) fluctuates in the range delta-P.sub.H in the upper part of the optical output waveform shown in FIG. 3C. Especially, as is shown in the curve (C), since the inclination of the curve (C) is sharp, the variation of the driving voltage is relatively amplified as a result of being converted into an optical output, in the upper part (C4). Thus, signal degradation occurs when a slight amount of ringing in an electrical driving waveform results in much higher levels of noise appearing in the upper part of the optical output. This results in a degradation of parameters such as optical receiver code-error rate and optical waveform.
Therefore, in a long-distance optical transmission system, the EAM driving apparatus in the related art has a problem in that it is not possible to utilize the advantages of an optical fiber transmission line sufficiently, which advantages are obtained as a result of its low-loss and wide-frequency-range characteristics.